Each year sudden cardiac death (SCD) claims an estimated 300,000 in the United States alone. An estimated 5-10% of these deaths occur in seemingly healthy individuals with otherwise structurally normal hearts following post-mortem investigation. Cardiac channelopathies such as Long-QT Syndrome (LQTS) and Brugada Syndrome (BrS) which arise from heritable defects in cardiac ion channel function represent the most common identifiable causes underlying autopsy negative sudden death, including 35% of Sudden Unexplained Death Syndrome (SUDS) and 10% of Sudden Infant Death Syndrome (SIDS) cases. While considerable effort has been devoted to understanding the pathogenesis of channelopathic sudden death, nearly 25% of LQTS, 70% of BrS, and a large proportion of autopsy negative sudden unexplained deaths still remain genetically elusive. While perturbations within the voltage-gated transient outward (Ito) current macromolecular complex have long been hypothesized to contribute to the pathogenesis of LQTS and BrS, there exists a relative paucity of molecular and functional evidence directly implicating genetic variation within primary Ito molecular determinants to disease. We hypothesize that mutations in the KCND3-encoded Kv4.3 1- subunit, KCNIP2-encoded KChIP2 2-subunit, or regulators of Ito channel expression such as microRNA-1-2 might lead to arrhythmic causes of sudden death. In support of this hypothesis we previously identified five potentially disease-associated non-synonymous mutations within KCND3 and KCNIP2 as well as a possible disease-associated nucleotide substitution within the stem of microRNA-1-2 in clinically robust, genotype negative LQTS (n=94) and BrS (n=91) cohorts. We will begin by assessing the full spectrum and prevalence of genetic variation within KCND3 and KCNIP2 in health and genetically elusive SCD by expanding our PCR/DHPLC-based mutational analysis to include 283 SIDS cases, 101 SUDS cases, and 780 ostensibly healthy controls. Next, to demonstrate that loss-of-function LQTS-associated mutations result in reduced Ito current and gain-of-function BrS-associated mutations result in increased Ito current, leading to prolonged or accelerated repolarization respectively we will functionally characterize all discovered mutations using the whole cell patch clamp technique. Finally, we will assess the functional impact of a disease-associated nucleotide substitution within microRNA-1-2 on Ito channel expression in the heart using quantitative RT-PCR and western blotting.